Microlithography is used for producing microstructured components, for example integrated circuits. The microlithographic process is performed using a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, currently under development are EUV lithography apparatuses that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 13.5 nm. In the case of such EUV lithography apparatuses, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, have to be used instead of—as previously—refractive optical units, that is to say lens elements.
The mirrors may for example be fixed to a supporting frame (force frame) and be designed as at least partially manipulable, in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the picometers range. This allows changes in the optical properties that occur for instance during the operation of the lithography apparatus, for example as a result of thermal influences, to be compensated for.
For mounting the mirrors on the supporting frame, usually weight force compensation units on the basis of permanent magnets (magnetic gravity compensators) are used, as described for example in DE 10 2011 088 735 A1. The compensation force generated by such a weight force compensation unit acts counter to the weight force of the mirror, and corresponds substantially to it in terms of its absolute value.
By contrast, the movement of a respective mirror—in particular also in the vertical direction—is actively controlled by way of so-called Lorentz actuators. Such a Lorentz actuator respectively includes an energizable coil and, at a distance from it, a permanent magnet. These together generate an adjustable magnetic force for controlling the movement of the respective mirror. By way of example, such Lorentz actuators are described in DE 10 2011 004 607 A1.
Sensors provided on a sensor frame can be provided for determining the position of the mirrors. With the aid of the sensors, the positioning of the mirrors can be checked and, if appropriate, corrected. The supporting frame and the sensor frame are mechanically decoupled from one another. A mechanical decoupling should be understood to mean that no forces, vibrations and/or oscillations are transmitted from the sensor frame to the supporting frame, or vice versa. This can be achieved for example via a very soft and/or resilient mounting of the sensor frame.
In order to prevent damage to the supporting frame and/or the sensor frame during tilting or during transport of the lithography apparatus, so-called end stops can be provided, which restrict a relative movement of the supporting frame with respect to the sensor frame and form a defined stop. Before the lithography apparatus is started up, a width of a gap provided at the end stops is checked with the aid of a feeler gauge. In this case, end stops that are not accessible and/or not visible can be checked only with difficulty or not at all. As an alternative to the use of a feeler gauge, the measurement of the width of the gap at the end stops can be carried out in a non-contact manner with the aid of an air pressure measuring method in which a nozzle through which air is guided is provided in the respective end stop. By determining the air pressure, it is possible to draw conclusions about the width of the gap.
If the gap at the end stops is too small, a mechanical short circuit can occur, that is to say touching contact between the supporting frame and the sensor frame. Such a mechanical short circuit can considerably disturb the dynamic behavior of a projection lens of the lithography apparatus. The correct determination of the width of the gap before the lithography apparatus is started up is therefore important for the reliable functionality of the lithography apparatus.